i really like the m80ti but with its 4 ohm load I would have to look for much more expensive amplification to power them optimally.

I was wondering what will I loose by going with the m60ti instead. Bass extension, dynamic headroom , and efficiency is a given but I am wondering if I will loose any detail/resolution ,soundstaging/imaging, midbass slam or anything else by going with the m60ti?

The M60 is an 8 ohm version of the M80. You will not lose any sound quality, imaging, or dynamics. The difference is in the clean acheivable output levels, but keep in mind that going for high clean output should be done by purchasing both the M80s and big power; either one in isolation will not achieve the goal.

I've ordered a pair of M80's on the belief that my modest NAD 304 integrated will be able to drive them. Your reference to high power being necessary to achieve high, clean output makes me wonder if I will need to upgrade the amp to hear the potential of the speakers? Or will the NAD be sufficient so long as I'm not trying to fill a stadium with sound?

You still will get an output gain from the M80s which will help, even with a
modestly sized receiver. What you will not be utilizing is the speaker's
actual full output potential. So you're right about stadium sound, but
these speakers will sound great immediately and will grow with your system.